docs: fix broken README links

[dajiaohuang]

What type of PR is this?

docs

Check the PR title.

• This PR title matches the format: <type>(optional scope): <description>
• The description is user-oriented and clear.
• This PR is itself the required documentation update.

(Optional) Translate the PR title into Chinese.

docs: 修复 README 中的失效链接

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

A full tracked-Markdown link check found 20 broken prose-link occurrences across 16 English and Chinese README files. This change:

• adds mailto: to four security email links
• corrects four ADK root-license links
• removes stale links to two example directories that do not exist
• corrects eight ACL links that were one directory level short

The English and Chinese README copies are updated together. The copied Markdown fixture under recursive splitter testdata is intentionally unchanged.

Validation:

• decoded all 245 tracked Markdown files as UTF-8 (0 decoding failures)
• checked 542 remaining local prose links
• 0 maintained-document failures; the only two remaining failures are inside the intentionally copied test fixture
• git diff --check

(Optional) Which issue(s) this PR fixes:

Fixes #964

(optional) The PR that updates user documentation:

This PR.